1. Field of the Invention
The present invention relates to an electrophotographic light-receiving member having a sensitivity to electromagnetic waves such as light (which herein refers to light in a broad sense and includes ultraviolet rays, visible rays, infrared rays, X-rays, .gamma.-rays, etc.), and also relates to a process for its production.
2. Related Background Art
In the field of image formation, photoconductive materials that form light-receiving layers in light-receiving members are required to have properties such that they are highly sensitive, have a high SN ratio [light current (Ip)/dark current (Id)], have absorption spectra suited to spectral characteristics of electromagnetic waves to be radiated, have a high response to light, have the desired dark resistance and are harmless to human bodies when used. In particular, in the case of electrophotographic light-receiving members set in electrophotographic apparatus used in offices, the harmlessness in their use is an important point.
Photoconductive materials having good properties in these respects include amorphous silicon hydrides (hereinafter "a-Si:H"). For example, U.S. Pat. No. 4,265,991 discloses its application in electrophotographic light-receiving members.
In such electrophotographic light-receiving members having a-Si:H, it is common to form photoconductive layers comprised of a-Si, by film forming processes such as vacuum deposition, sputtering, ion plating, heat-assisted CVD, light-assisted CVD and plasma-assisted CVD while heating conductive supports at 50.degree. C. to 350.degree. C. In particular, the plasma-assisted CVD, i.e., a process in which material gases are decomposed by direct-current, high-frequency or microwave glow discharging to form a-Si deposited films on the support, has been put into practical use as a preferred process.
German Patent Application Laid-open No. 30 46 509 discloses an electrophotographic light-receiving member having an a-Si photoconductive layer containing a halogen atom as a constituent (hereinafter "a-Si:X"photoconductive layer). This publication reports that incorporation of 1 to 40 atom % of halogen atoms into a-Si enables achievement of a high thermal resistance, and also electrical and optical properties preferable for a photoconductive layer of an electrophotographic light-receiving member.
Japanese Patent Application Laid-open No. 57-115556 also discloses a technique in which a surface barrier layer formed of a non-photoconductive amorphous material containing silicon atoms and carbon atoms is provided on a photoconductive layer formed of an amorphous material mainly composed of silicon atoms, in order to achieve improvements in photoconductive members having a photoconductive layer formed of an a-Si deposited film, in respect of their electrical, optical and photoconductive properties such as dark resistance, photosensitivity and response to light and service environmental properties such as moisture resistance and also in respect of stability with time. U.S. Pat. No. 4,659,639 discloses a technique concerning a photosensitive member superposingly provided with a light-transmitting insulating overcoat layer containing amorphous silicon, carbon, oxygen and fluorine. U.S. Pat. No. 4,788,120 discloses a technique in which an amorphous material containing silicon atoms, carbon atoms and 41 to 70 atom % of hydrogen atoms as constituents is used to form a surface layer.
U.S. Pat. No. 4,409,311 discloses that a highly sensitive and highly resistant, electrophotographic photosensitive member can be obtained by using in a photoconductive layer an a-Si:H containing 10 to 40 atom % of hydrogen and having absorption peaks at 2,100 cm.sup.-1 and 2,000 cm.sup.-1 in an infrared absorption spectrum which peaks are in a ratio of 0.2 to 1.7 as the coefficient of absorption.
Meanwhile, U.S. Pat. No. 4,607,936 discloses a technique in which, aiming at an improvement in image quality of an amorphous silicon photosensitive member, image forming steps such as charging, exposure, development and transfer are carried out while maintaining temperature at 30 to 40.degree. C. in the vicinity of the surface of the photosensitive member to thereby prevent the surface of the photosensitive member from undergoing a decrease in surface resistance which is due to water absorption on that surface and also smeared images from occurring concurrently therewith.
These techniques have achieved improvements in electrical, optical and photoconductive properties and service environmental properties of electrophotographic light-receiving members, and also have concurrently brought about an improvement in image quality.
The electrophotographic light-receiving members having a photoconductive layer comprised of an a-Si material have individually achieved improvements in properties in respect of electrical, optical and photoconductive properties such as dark resistance, photosensitivity and response to light and service environmental properties and also in respect of stability with time, and running performance (durability). Under existing circumstance, however, there is room for further improvements to make overall properties better.
In particular, there is rapid progress in making electrophotographic apparatus with higher image quality, higher speed and higher running performance, and the electrophotographic light-receiving members are required to have improved in electrical properties and photoconductive properties and also to maintain their running performance over a longer period of time in every environment while maintaining charge performance and sensitivity.
Then, as a result of improvements made on optical exposure devices, developing devices, transfer devices and so forth in order to improve image characteristics of electrophotographic apparatus, the electrophotographic light-receiving members are now also required to be more improved in image characteristics than ever.
Under such circumstances, although the conventional techniques as noted above have made it possible to improve properties to a certain degree in respect of the subjects stated above, they can not be said to be satisfactory in regard to the further improvements in charge performance and image quality. In particular, as the subjects for making amorphous silicon light-receiving members have much higher image quality, it has now become more desirable to decrease exposure memory such as blank memory and ghost.
For example, hitherto, in order to prevent smeared images caused by photosensitive members, a drum heater for keeping a photosensitive member warm is set inside a copying machine to keep the surface temperature of the photosensitive member at about 40.degree. C., as disclosed in U.S. Pat. No. 4,607,936. In conventional photosensitive members, however, the dependence of charge performance on temperature, called temperature-dependent properties, that is ascribable to the formation of pre-exposure carriers or heat-energized carriers is so great that, in the actual service environment inside copying machines, photosensitive members could not avoid being used in the state where they have a lower charge performance than that originally possessed by the photosensitive members. For example, the charge performance may drop by nearly 100 V in the state where the photosensitive members are heated to about 40.degree. C. by a drum heater, compared with the case when used at room temperature.
At night when copying machines are not used, the drum heater is kept electrified in conventional cases so as to prevent the smeared images that are caused when ozone products formed by corona discharging of a charging assembly are adsorbed on the surface of a photosensitive member. Nowadays, however, it has become popular not to electrify copying machines at night for the purpose of saving natural resources and saving electric power.
When copies are continuously taken in such a state, the surrounding temperature of the photosensitive member inside a copying machine gradually rises to make charge performance lower with a rise of the temperature, causing the problem of a change in image density during the copying.
Namely, when the photosensitive member is continuously used, the surface temperature thereof rises as a result of charging and exposure to cause a lowering of charge performance, resulting in a change in image density during the copying to cause a lowering of image quality. Hence, in order to mount it in an ultra-high speed machine (copying on, e.g., 80 sheets or more per minute), it is necessary to decrease the temperature-dependent properties.
Meanwhile, in conventional photosensitive members, when the same original is continuously and repeatedly copied, a decrease in image density may occur or fog may occur because of exposure fatigue of photosensitive members as a result of imagewise exposure.
For example, when the same original is continuously and repeatedly copied, a change in image density (gradual increase or decrease in density) may occur because of accumulation of carriers or accumulation of charged carriers as a result of exposure (i.e., charge potential shift in continuous charging).
The exposure memory such as blank memory and what is called ghost have also come into question for the improvement of image quality; the blank memory being a phenomenon which causes a density difference on copied images, caused by what is called blank exposure that is applied to the photosensitive member at paper feed intervals during continuous copying in order to save toner, and the ghost being a phenomenon in which an image remaining after the imagewise exposure in previous copying (after-image) is produced on an image in the subsequent copying.
From the viewpoints of preventing the exposure memory, making an apparatus smaller in size, and considering ecological problems and saving energy, there is a demand for imagewise exposure assemblies having a smaller amount of exposure and a smaller size. Improvements in photosensitivity of photosensitive members, however, must be further advanced in order to meet such a demand.
In addition, in conventional photosensitive members, when the amount of exposure is increased so that an image with a strong contrast can be obtained from a color-background original, photo-carriers are produced in a large quantity because of application of intense exposure to cause a phenomenon in which the photo-carriers gather to and flow into portions to which they can readily move. This phenomenon has caused the problem of smeared images in intense exposure, what is called smeared EV, which causes blurred letters or characters.
Accordingly, in designing electrophotographic light-receiving members, it is required to achieve improvements from the overall viewpoints of layer configuration and chemical composition of each layer of electrophotographic light-receiving members so that the problems as discussed above can be solved, and also to achieve more improvements in properties of the a-Si materials themselves.